.TH std::ranges::reverse 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::ranges::reverse \- std::ranges::reverse

.SH Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::bidirectional_iterator I, std::sentinel_for<I> S >

   requires std::permutable<I>                                       \fB(1)\fP \fI(since C++20)\fP
   constexpr I

       reverse( I first, S last );
   template< ranges::bidirectional_range R >

   requires std::permutable<ranges::iterator_t<R>>                   \fB(2)\fP \fI(since C++20)\fP
   constexpr ranges::borrowed_iterator_t<R>

       reverse( R&& r );

   1) Reverses the order of the elements in the range [first, last).
   Behaves as if applying ranges::iter_swap to every pair of iterators first + i, last
   - i - 1 for each integer i, where 0 ≤ i < (last - first) / 2.
   2) Same as \fB(1)\fP, but uses r as the range, as if using ranges::begin(r) as first and
   ranges::end(r) as last.

   The function-like entities described on this page are niebloids, that is:

     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.

   In practice, they may be implemented as function objects, or with special compiler
   extensions.

.SH Parameters

   first, last - the range of elements to reverse
   r           - the range of elements to reverse

.SH Return value

   An iterator equal to last.

.SH Complexity

   Exactly (last - first) / 2 swaps.

.SH Notes

   Implementations (e.g. MSVC STL) may enable vectorization when the iterator type
   models contiguous_iterator and swapping its value type calls neither non-trivial
   special member function nor ADL-found swap.

.SH Possible implementation

   See also implementations in libstdc++ and MSVC STL.

   struct reverse_fn
   {
       template<std::bidirectional_iterator I, std::sentinel_for<I> S>
       requires std::permutable<I>
       constexpr I operator()(I first, S last) const
       {
           auto last2 {ranges::next(first, last)};
           for (auto tail {last2}; !(first == tail or first == --tail); ++first)
               ranges::iter_swap(first, tail);
           return last2;
       }

       template<ranges::bidirectional_range R>
       requires std::permutable<ranges::iterator_t<R>>
       constexpr ranges::borrowed_iterator_t<R>
           operator()(R&& r) const
       {
           return (*this)(ranges::begin(r), ranges::end(r));
       }
   };

   inline constexpr reverse_fn reverse {};

.SH Example


// Run this code

 #include <algorithm>
 #include <array>
 #include <iostream>
 #include <string>

 int main()
 {
     std::string s {"ABCDEF"};
     std::cout << s << " → ";
     std::ranges::reverse(s.begin(), s.end());
     std::cout << s << " → ";
     std::ranges::reverse(s);
     std::cout << s << " │ ";

     std::array a {1, 2, 3, 4, 5};
     for (auto e : a)
         std::cout << e << ' ';
     std::cout << "→ ";
     std::ranges::reverse(a);
     for (auto e : a)
         std::cout << e << ' ';
     std::cout << '\\n';
 }

.SH Output:

 ABCDEF → FEDCBA → ABCDEF │ 1 2 3 4 5 → 5 4 3 2 1

.SH See also

   ranges::reverse_copy creates a copy of a range that is reversed
   (C++20)              (niebloid)
   ranges::reverse_view a view that iterates over the elements of another bidirectional
   views::reverse       view in reverse order
   (C++20)              \fI(class template)\fP (range adaptor object)
   reverse              reverses the order of elements in a range
                        \fI(function template)\fP
